In the prior art, curing resin compositions are known comprising a hydroxyl group-containing resin such as an acrylic resin or polyester resin, blended with a melamine resin or polyfunctional isocyanate compound as a curing agent. However, in such melamine curing systems, curing requires a high temperature of 150.degree.-230.degree. C. resulting in poor workability and economic viability. They also lose lower alcohols with a corresponding loss of volume. Further, they suffer from the disadvantage of poor weatherability due to the triazine skeleton, and in the case of isocyanate curing systems, there was also the problem of toxicity and decline of weatherability.
Methods have been reported to improve weatherability by condensation crosslinkage at room temperature of acrylic resins wherein alkoxysilyl groups are introduced into the molecule (e.g., Japanese Kokai Koho (Unexamined Publication) Nos. 57-36109 and 58-55666), but as their curing rate was slow, contamination occurred due to hydrolysis of remaining alkoxysilyl groups and they also had poor anti-corrosion properties. Other curing systems have been reported which make use of the reactivity of alkoxysilyl groups and hydroxyl groups in organic resins (e.g., Japanese Kokoku Koho (Examined Publication) No. 63-33512); however, not only their thermocuring rate was slow, but also their resistance to salt water was poor.
On the other hand, systems where a vinylpolysiloxane and organohydrogenpolysiloxane were cured in the presence of a platinum catalyst have been known for many years.
However, the cured product swells up in solvents and therefore has poor solvent resistance. In addition, it has poor alkali resistance and as it also has poor recoatability, its use in the paint field has been very limited.
The improvement of recoatability by using alkoxysiloxane-modified polyesters containing propyl groups (e.g., Japanese Patent Kokai Koho No. 62-263265), and crosslinking methods by using polysiloxanes containing alkoxysilylalkyl groups (e.g., Japanese Kokai Koho Nos. 57-139123 and 61-127733), have been proposed. Even using these methods; however, a curing composition which is satisfactory from the viewpoints of mechanical strength, compatibility with other resins and recoatability, has still not been found.
Further, in recent years, paints which contain organic solvents have been cited as factors responsible for atmospheric pollution.
The inventors carried out intensive studies to resolve these disadvantages in the prior art and found that by crosslinking an organic resin containing unsaturated groups using the addition reaction of an organohydrogenpolysiloxane, the weatherability and physical properties, i.e., solvent resistance, alkali resistance, acid resistance, water resistance, salt water resistance, anti-corrosion properties and contamination resistance, could be improved. In addition, recoatability was satisfactory, and when the composition was used as a paint, there was not necessarily any need to use organic solvents.